It's relatively easy if you are flexible! A few weeks ago I called mom up and asked if she wanted to go to Europe for the weekend. We decided on Munich, saw that the flights were fulling up towards a week before, so we opted on Edinburgh instead. Great flights both ways...got business elite too each way. Coming back though was a bit stressful in that the Glasgow airport bombing occured that weekend, so it seemed that lots of people wanted to go back home, and the flight back to ATL oversold by 36 people. We stayed an extra day and it was no problem, however our contingency was to take the train to Manchester and fly out which was an open flight.
As Kristie said its a tad more predictable going over the pond because the loads do indeed steady out about 2-3 days out from your trip. I've had domestic flights fill up and oversell between me checking in the morning and heading to the airport in the afternoon!
Not sure how USAirways is, but I would definately recommend using the benefits to your advantage to see Europe. It's a little stressful when you are at the airport and things seem a bit more chaotic than in the States, but when you are sitting in your business class seat (or economy for that matteR) and you know you only paid about 80-100 dollars for the trip, IT'S WORTH IT!
One thing, and I've noticed this in several destinations (EDI, CDG-the worst, ATH, and FCO) that when you are standby sometimes they don't let you through security until checkin has closed. That's when they sometimes clear standbys instead of loading your bags and letting you proceed through security with only a seat request card. In short, it's a free for all when they start shouting last names in a crowd full of people.
Enjoy your travels though...I know I do!